The present invention relates to a sensor element, particularly to a sensor such as a magiletoresistance sensor, an air flow sensor, an acceleration sensor, a pressure sensor, a yaw rate sensor, or an image sensor having a constant area sensor face.
Conventionally, an acceleration sensor, a yaw rate sensor, a pressure sensor, an air flow sensor, and a magnetoresistance sensor are used as sensor elements for controlling running of a vehicle. Among them, each of the acceleration sensor, the yaw rate sensor, and the pressure sensor includes a flat pivotally moving electrode (sensing portion) responding to impact or acceleration, and detecting a change in electric capacitance between the electrode and an opposed electrode fixedly arranged proximate the moving electrode. Further, various metal materials are used for the planar electrode constituting the sensing portion, for example, as described in Japanese Patent Laid-Open No. Hei. 5-183145. Japanese Patent Laid-Open No. Hei. 5-283712, or Japanese Patent Laid-Open No. Hei. 6-194382, a surface thereof is covered and protected by a silicon nitride film or a silicon oxide film, and these inorganic thin films are formed by sputtering, CVD, or another vapor deposition process.
However, the planar electrode is provided with a three-dimensional structure projected from a supporter for supporting the electrode in a plane direction (thickness is normally about 10 through 500 xcexcm) and accordingly, coverage performance of an inorganic thin film by the vapor deposition process or the CVD process is poor, particularly, it is difficult to form the inorganic thin film at side faces of the planar electrode and there is a concern that protection performance is deteriorated.
Further, although an air flow sensor for detecting a flow rate of gasoline is constituted to detect a flow rate of a gas including gasoline by detecting a temperature variation of a sensing portion produced by bringing the sensing portion in a planar shape embedded with a resistance wiring into contact with a flow path of the gas including gasoline by a change in resistance of the resistance wiring, since the sensor is of a three-dimensional structure having a large stepped difference between the sensing portion and a supporter, with regard to protection of the sensing portion, a problem similar to the above-described is posed.
Therefore, it is conceivable to thicken the inorganic thickness to sufficiently protect side faces of the planar electrode by the inorganic thin film, however, high stress is caused at the sensing portion and crack is caused in the film per se. Thereby, there poses a problem that the sensor characteristic is deteriorated or a position shift is caused in the resistance wiring constituting the sensor portion or a wiring on a contiguous control circuit. Particularly, in the case of a sensor in which the wiring of the resistance wiring is used for the sensing portion, as the wiring, there is used a special material which is not used in a normal semiconductor element and therefore, depending on the constituting material, adherence with a substrate material is significantly weak, for example, when the sensor element is sealed by a resin, there poses a problem that such a wiring is liable to cause the positional shift by thermal or mechanical strain.
It is conceivable to use polyimide resin or photosensitive polyimide resin on sale as the protective film of the sensing portion, however, in view of stress characteristic, heat resistance, degasing performance and compatibility with fabrication process (processing temperature or the like), there is frequently a case in which adverse influence is effected on the characteristic of the sensing portion. That is, formation of a film of polyimide resin is carried out by chemical reaction in which polyimide is formed from amic acid monomer by chemical reaction and accordingly, residual stress is high and contamination is caused by gas components produced by the reaction. Further, a polyimide resin protective film is inferior in water proof performance and environment of using the sensor element is restricted.
Meanwhile, different from the above-described sensor having the three-dimensional structure, a magnetoresistance sensor is not basically provided with a significant stepped difference, provided with a sensing portion in a planar shape comprising slender wires of magnetic metal, constituted to be capable of detecting a change in magnitude or direction of a magnetic field by utilizing a magnetoresistance effect of a magnetic metal and although it is conceivable to protect the sensing portion by a polyimide resin film, however, it is necessary to carry out a curing processing at high temperature, the curing temperature is higher than heat resistance limit temperature of the main body of the sensor and there poses a problem that sufficient curing processing cannot be carried out.
The invention has been carried out under such a situation and it is an object thereof to provide a sensor element in which particularly, a sensing portion in a planar shape is coated with a protective film excellent in step coverage, having low stress applied to the sensing portion, capable of being formed at low temperature and having no concern of effecting adverse influence to the sensing portion even in a fabrication process.
According to the invention, there is provided a sensor element having a sensor substrate and a flat sensing portion supported by the sensor substrate, wherein the surface of the flat sensing portion is covered with a silicone resin film.
The silicone resin film used in the invention is provided with heat resistance against temperature in a process of fabricating the sensor element or used temperature, excellent in coverage performance, excellent in coverage performance of the flat sensing portion having the three-dimensional structure, provided with low stress performance, can be thickened and provided with high environment resistance. Further, according to the silicone resin film, different from polyimide resin used as a protective film in a semiconductor device, an amount of generating gas in curing the film is extremely small, and contamination is not caused not only at the sensing portion but also at a contiguous control circuit portion. Further, the silicone resin film exceeds polyimide resin in heat resistance, adhering performance and low stress performance. Further, the silicone resin film is more excellent than polyimide resin or a material of an inorganic film formed by a conventional CVD process or sputtering process in view of coverage performance.
The silicone resin film can be formed by coating a solution of the silicone polymer and heating and curing thereof. According to such forming method, by rotationally coating and heating thereof, the sensing portion can be covered and protected simply and effectively and adverse influence effected to the sensing portion as in the polyimide species protective film is resolved. Further, there is not used an expensive film forming apparatus as in an inorganic species protective film, which is advantageous in view of fabrication cost. Therefore, according to the invention, there is provided a method of fabricating a sensor constituted by covering the sensing portion by the silicone resin film by coating the solution of the silicone polymer to the flat sensing portion supported by the sensor substrate and heating and curing thereof.